Run-Time Check Failure #2 - Stack around the variable 'a' was corrupted - C (СИ)

main()
{
    int i, j, c, b, g, K = 0, time = 0;
 
    const int N = 6, M = 25;
 
    char a[N][M] = { { '_','_','_','_', '_', '_', '_', '_', '_', '_', '_','_', '_','_', '_', '_', '_', '_', '_','_', '_','_','_', '_', },
                            { '|',' ',' ',' ', ' ',' ',' ', ' ', '|', ' ',' ',' ', ' ',' ', ' ', ' ', '|',' ',' ', ' ',' ', ' ',' ', ' ','|', },
                            { '|',' ',' ',' ', '1',' ',' ', ' ', '|', ' ',' ',' ', '2',' ', ' ', ' ', '|',' ',' ', ' ','3', ' ',' ', ' ','|', },
                            { '|',' ',' ',' ', ' ',' ',' ', ' ', '|', ' ',' ',' ', ' ',' ', ' ', ' ', '|',' ',' ', ' ',' ', ' ',' ', ' ','|', },
                            { '|',' ','  ',' ', '  ','  ','  ', '  ', '|', ' ',' ',' ', ' ',' ', ' ', ' ', '|',' ',' ', ' ',' ', ' ',' ', ' ','|', },
                            { '|','_','_', '_','_','_', '_', '_','|','_', '_','_','_', '_', '_', '_', '|','_','_','_', '_', '_','_', '_','|', } };
 
    for (g = 0; g < 7; g++)
    {
        switch (g)
        {
 
        case 0: time = 20; K = 80; break;
 
        case 1: time = 50; K = 32; break;
 
        case 2: time = 100; K = 20; break;
 
        case 3: time = 200; K = 10; break;
 
        case 4: time = 400; K = 4; break;
 
        case 5: time = 600; K = 2; break;
 
        }
 
        for (b = 0; b < K; b++)
        {
            printf("           ________\n");
            printf("           \\      //\n");
            printf("            \\    //\n");
            printf("             \\  //\n");
            printf("              \\//\n");
 
            for (i = 0; i < N; i++)
            {
                printf("\n");
 
                for (j = 0; j < M; j++)
                {
                    a[i][25] = a[i][1];
 
                    a[i][j] = a[i][j + 1];
 
                    printf("%c", a[i][j]);
                }
            }
 
            Sleep(time);
            system("cls");
        }
 
    }
    system("pause");
    return 0;
}